The standard delivery for local anesthesia use in dentistry and medicine is with the anesthetic cartridges (carpules). In the medical, dental and veterinary fields, cartridge ampoules of injectable fluid are now commonly used in hypodermic syringes. Such ampoules usually comprise a cylindrical container of glass or clear plastic material, the forward end of which is sealed by a rubber membrane and the rear end of which is sealed by a rubber piston which is slideable within the container. They are manufactured under rigid standards under which the anesthetic reaches the dentist as sterile ampoules. In use, such cartridge ampoules are positioned within the barrel of a syringe so that the membrane is pierced by the rear end of a needle and the rubber piston is advanced into the container by manual thumb force against a thumb-operated plunger to inject the fluid through the needle into the patient.
In addition to injecting the liquid into the patient, it is important in many instances for the syringe to be used for aspiration. Aspiration occurs when the doctor retracts the plunger gently after the needle is inserted in the tissue to ensure that it is not in a blood vessel. That is, if blood is aspired or drawn back through the needle, the doctor will see this and remove the needle from that location and attempt an injection in a different location.
It is greatly desired when administering an anesthetic via a syringe to be able to use precise control of the syringe so as not to inject the anesthetic too quickly. Rapid injection may result in tissue damage, or at least great discomfort to the patient. Thus, a slow and precise control over the injection is required, wherein excess pressure is avoided. It is generally very difficult to control the flow of anesthetic, however, with a great degree of accuracy and finesse. This is because the plunger pushes directly on the rubber stopper within the cartridge. This creates a direct hydraulic pressure on the liquid to be dispensed, which forces the liquid out of the needle with excessive force. As a pocket of anesthetic solution builds up within the tissue, it has nowhere to go until it is absorbed in the surrounding tissues.
Disposable syringes are plastic syringes that have the anesthetic liquid disposed directly in a dispensing chamber rather than utilizing a separate cartridge. The principles of liquid delivery and the need for precise control are the same as with the cartridges described above.
It is therefore an object of the present invention to provide an anesthetic cartridge or disposable syringe that facilitates smoother and gentler delivery of its contents.
It is desired to be able to implement such precise control of the delivery of anesthetics without requiring an excessive number of parts, such as springs or valves, which would complicate manufacturing.
It is further desired to provide a traditional anesthetic cartridge or disposable syringe with the innovative modification without compromising quality, safety or simplicity and would be inexpensive to manufacture.
It is also desired to provide such a device that selectively permits the plunger to engage the rubber stopper within the cartridge to move longitudinally in either direction to inject or aspirate in a normal fashion.
It is also an object of the present invention to provide a means for the backpressure that is created at the injection site to be controlled, to cause as little pain or discomfort as possible.